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96 PCB007 MAGAZINE I JUNE 2020 Machines that have been made to UV cure inks used in printed circuits and other manu- facturing processes have largely used vapour lamps as the UV light source since the earliest development of UV-cured materials. Typically, vapour bulbs contain a proportion of mercury, which is in the process of undergoing a world- wide ban. In turn, this means that alternative UV light sources with enough power to com- plete the UV curing process have a very clear commercial interest. The development of the photoinitiators, which are the components in UV ink that en- able cure when exposed to UV light, was based on the best UV source available at the time. If you want to guarantee that you will achieve exactly the same result when you switch light sources, you will need to accurately mimic the output of the vapour bulb in both spectrum and intensity. In the search for good alternatives, LED tech- nology has been hovering in the background, but for a number of reasons, it has struggled to position itself as a serious contender. UV LEDs have worked well for the photo- image exposure of dry film and solder mask, and there are quite a number of established machines available for this purpose. I have set a few of these machines up at customer factories, and the empirical test methods— such as using step wedges to control the ex- posure power—work much better when as- sessing the output of the LEDs. The good results from the LED exposure machines sug- gest that the same LED method should be suitable for other UV light processes, such as bump of solder mask (additional exposure after developing the photoimageable mask) to make the ink more robust for the aggres- sive chemical and heat processes that some LED UV Cure: Does It Really Work? Ladle on Manufacturing by Marc Ladle, VIKING TEST LTD.

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